The present invention relates to an electron beam gun for an electron beam evaporation source of the type used in depositing a thin film of evaporated material onto a substrate. More particularly, the present invention relates to such an electron beam gun in which a beam former used in generating a ribbon-like electron beam to melt and evaporate the material is formed in two sections that are separately connected to a split cathode block to prevent thermal deformation of the beam former. The prevention of thermal deformation of the beam former increases the service life the electron beam gun.
In the prior art, electron beam evaporation sources employ an electron beam gun located beneath a crucible containing a material to be evaporated. The evaporated material is subsequently deposited onto a substrate held above the evaporation source. In such evaporation sources, the electron beam gun emits a ribbon-like beam that is magnetically deflected through an arc of 270.degree. and into the crucible to melt and evaporate the material.
The electron beam gun employs a filament connected to a split cathode block having two sides. An anode having a top section of `U` shaped cross-section passes over the filament and terminates in a front edge spaced from and located in front of the filament. The beam former is located between the anode and the filament and is configured so that the filament is shielded from the anode. When an electrical current is applied to the filament through the cathode block, the filament emits electrons which because of the shielding of the beam former are constrained to pass beneath the beam former and toward the front edge of the anode in a ribbon-like electron beam. By the time the electrons reach the anode a sufficient kinetic energy has been imparted to the electrons to cause the electrons to miss the anode and thereby to be emitted from the electron beam gun. The ribbon-like beam emitted by the electron beam gun is deflected by magnetic pole pieces of the evaporation source into the crucible.
The beam former, which has essentially the same shape of the anode, is connected to one of the sides of the cathode block and insulated from the other of the sides of the cathode block by an insulator. The cathode block acts as a heat sink to the beam former, but since the beam former is only connected to one side of the cathode block, a thermal gradient is produced across the beam former. The resultant unequal heat distribution within the beam former causes the beam former to warp and move. Such warpage and movement can cause the beam former to rise above the lower front edge of the anode and thus cause electrons to directly arc into the anode, destroying the anode. In addition, the warpage and movement of the beam former can deform the beam. Such deformation can change the impact area, and thus, produce a non-uniform evaporation of the material to be evaporated.
As will be discussed, the present invention provides an improved electron beam gun in which the beam former and its attachment to the cathode block are designed to prevent the production of thermal gradient across the beam former to in turn prevent warpage and movement of the beam former.